1. Field of the Invention
Electrostatic spraying devices which provide spray streams of charged liquid particles by an induction-charging mechanism are well known. Of particular interest herein is an adapting device that provides means for mounting an induction-charging adapter head to a spray device and for quickly and safely disconnecting the adapter head from the spray device.
2. State of the Art
There has been recently provided an improved electrostatic spray device of the induction-charging type as disclosed in U.S. Pat. No. 4,009,829 to J. E. Sickles and in U.S. Patent Application Ser. No. 814,953 filed July 12, 1977 of J. E. Sickles, the disclosures of which are incorporated herein by reference. These induction-charging electrostatic spray devices are characterized in having an induction-charging electrode disposed exteriorly of, or outwardly from, an external-mixing liquid spray-forming nozzle.
Electrostatic induction-charging external-mixing spray devices, especially of the hand-held type, provide significant advantages in safety and in coating deposition efficiency over corona or contact charging spray devices. Corona-discharge spraying systems typically utilize needle-like electrodes that establish corona-producing electric fields by application of potentials of about 100,000 volts to the electrode with resulting corona-discharge currents approximating 50-300 microamps. Such high-power electric discharges present potential shock hazards to equipment operators. Moreover, there is great likelihood in corona systems of high potential electric discharge by arcing from the electrode to a ground point or by sparks from the electrode to air-borne particulate matter, which electric discharges can ignite flammable paint vapors. The hazard of fire and explosion resulting from paint vapors ignited by arcing or sparking produced by corona-discharge equipment has, for example, substantially precluded use by major household appliance manufacturers of electrostatic spray devices for spraying organic-based paints onto the interior surfaces of appliance cabinets.
An induction-charging spray device of the aforementioned type provides, under ideal conditions, practically no likelihood of substantial arcing, sparking, or high energy corona discharges. The absence of any substantial discharge is assured by an electrode surface configuration that is devoid of sharp edges and points and by the application of high voltage potentials to the electrode of about 25,000 volts or less, with normal current dissipation by the electrode being at a level of about 1 to 3 microamps or less. With the induction-charging electrode operating at these substantially lower voltage and current levels as compared to a typical corona-discharge electrode, any incidence of arcing or sparking is substantially reduced. Moreover, operator injury resulting from electric shock is avoided by the practically insignificant current available to be delivered by the electrode.
In addition to the aforementioned improved safety features, the described induction-charging spray device provides improved charged particle atomization. It has been found that a spray device comprising an induction-charging electrode disposed exteriorly of, or outwardly from, an external-mixing nozzle provides an assembly of particles characterized by a high degree of fineness and uniform size and having a relatively high average charge-to-mass ratio. These factors are important in achieving maximum transfer of coating material from the spray device to the target substrate and for achieving levelling or flow of the material into an evenly deposited, uniformly coalesced film.
This unique combination of safety and deposition efficiency features of the described induction-charging spray device is responsible for the significant commercial success of the device in over-coming problems inherent with corona-charging types of electrostatic spray equipment.
Another quite significant advantage of the aforementioned induction-charging systems is that an induction-charging adapter head of the described type is adaptable to many conventional spray devices, both of the electrostatic and non-electrostatic types. Thus a non-electrostatic spray device such as the Binks Model 62 external-mixing, hand-held spray gun (Binks Mfg. Co., Chicago, Ill.) may have attached to its forward barrel portion an induction-charging adapter head of the type disclosed in aforementioned U.S. Pat. No. 4,009,829. This combination of spray-forming means and induction-charging means has proven particularly suitable for many industrial applications where hand-held electrostatic spray devices are used to spray organic based flammable paints in confined spaces.
It has been found, however, that after periods of spraying of pigmented paints or high solids materials in confined spaces, the induction-charging electrode of an induction-charging system may collect significant amounts of coating material. It may then be necessary to detach the induction-charging adapter head from the spray device so that the contaminated adapter head may be soaked in a container of solvent for a period of time to remove the accumulated coating material. A spare adapter head may then be mounted on the spray device so that the spraying operation may continue. It is desirable, in order to minimize the length of equipment down-time, that mounting or demounting of an adapter head onto, or from, a spray device be effected quickly.
It is known that effective particle formation and charging is accomplished by the balancing of several parameters, such as, the velocity of liquid flow, the viscosity and electrical conductivity of the liquid material to be atomized, the velocity of the atomizing air flow, the charging voltage applied to the induction-charging electrode, the radial distance of the induction-charging electrode outwardly from the axis of the spray-forming nozzle, and the axial location of the electrode with respect to the plane of the nozzle discharge ports. A change in any one of these parameters may require some compensating alteration in one or more other parameters. It is highly desirable, therefore, that in the midst of a spraying operation after adjustment of other spraying parameters, the replacement of an induction-charging adapter head be accomplished such that the replacement head-to-spray gun relative position is virtually identical to that of the original head-to-gun position.
Another problem associated with the mounting and demounting of an induction-charging adapter head to and from a spray gun is the possibility of electrical shock to an equipment operator. For example, there may occur a failure in an induction charging system equipment component, such as a power-controlling flow switch failing to interrupt power to the induction charging electrode between spraying cycles. And frequently an equipment operator when removing an adapter head from a spray device will grip or touch the charging electrode. If the ground connection is broken between electrical ground and an induction-charging head of the type having a ground shield, the body of an operator may then form a segment of the path from the ground shield to electrical ground. Contact by the operator with the charged electrode after the ground connection is broken may result in unpleasant electrical shock to the operator.